Author's Response to Discussion of "SAGD and Geomechanics"

Abstract

I would like to thank Mr. Li and Dr. Chalaturnyk for their comments. During the preparation of the article, I concentrated on providing the basic theory and application of soil mechanics to SAGD, on the assumption that most readers would not be intimately familiar with the material. Mr. Li's and Dr. Chalaturnyk's discussion addresses more detailed points with respect to SAGD. Overall, I agree with the substance of most of their points. My objective in this reply is to provide some extension to his comments or provide clarifications. I have addressed the issues following the outline of their main point. Interactions of Sand Grains The points that Mr. Li and Dr. Chalaturnyk have made regarding the failure style are completely correct. The practical implication of their comments is that different lab data should be input for the different zones based on tests that are similar to the conditions that will exist in the reservoir during SAGD. While I agree with the concepts they have brought up, there arc other relevant factors. Change and Timing of Conditions The style of failure in certain parts of the reservoir may change with time as the SAGD process progresses. In this case, it can be difficult to change the input in the simulator with time. In these circumstances, it is common to pick the data available or that best represents the dominant mechanism. Variations in Lithology and Constitutive Model The geotechnical models for combined reservoir-geomechanical modelling are normally based on a single constitutive (stress-strain relation) model. The most common are clasto-plastic or hyperbolic type models. At present, many of these models do not allow for different mechanisms in different parts of the reservoir (or in the overburden). For instance, the behaviour of shales or mudstones in overburden is different than sands in the reservoir. This can limit the ultimate accuracy of any geomechanical model. Data Screening and Availability Complete data representing all facies and all areas is difficult to obtain and analyze. At present, there are relatively few samples that have been rigourously analyzed. Like other conventional reservoir data, there is often significant variation in samples and it is rare that average or representative properties are known with certainty. Initial Stress Conditions A number of papers that I have read on this topic have assumed a uniform or isotropic horizontal stress field; i.e. the same in both the x and y directions. There are sensitivities typically conducted that includt different magnitudes of horizontal stresses relative to vertical stresses (but, again, assumed to be horizontally isotropic). The work that Applied Reservoir Engineering (ARE) has most recently done is based on anisotropic stress states due to the effects of tectonics. Indeed, isotropic conditions, even restricted to the horizontal plane, are unlikely; hence, anisotropic conditions are a more realistic assumption. IT is also the case that the shearing is more likely to occur under such conditions. The basis and source of this analysis is outlined in Collins. P.M.,"Injection Pressures for Geomechanical Enhancement of Recovery Processes in the Athabasca Oil Sands" (1).